This invention relates to a ferrous-metal detector of the kind having a magnet with a Hall-effect element attached, and more particularly to such a detector having two spaced integrated circuit Hall sensors attached to a magnet.
When a ferromagnetic article passes by a magnet-biased Hall-effect element the Hall voltage changes due to the distortion of the magnet bias field. However, with no ferromagnetic article nearby, the Hall element output voltage is a large steady value. Output signal voltages are always superimposed on this large DC voltage. It is therefore customary to capacitor-couple or otherwise AC-couple the output of such detectors to an amplifier and/or another signal responsive device. Besides the extra expense of providing such an AC coupling, the system can only respond to moving ferromagnetic articles that must travel at least fast enough that the AC coupling means can follow it without substantial attenuation.
A more satisfactory solution to this problem is a circuit means for compensating the bias field as is described in the copending patent application to Avery and Higgs Ser. No. 342,687, filed Jan. 26, 1982, now U.S. Pat. No. 4,465,976, and assigned to the same assignee as is the present invention. A current is introduced at one or both output pads of the Hall-element to compensate for the bias field by opposing the generation of an output voltage due to the bias magnet. However, the sensitivity of this device is, as are most Hall-devices, a function of temperature.
The silicon integrated-circuit magnetic-field detector having a Hall-element and a Hall-voltage amplifier is described in the patent to Anselmo et al U.S. Pat. No. 3,816,766 issued June 11, 1974 and assigned to the same assignee as is the present invention. The Hall-element and the collector resistors in the differential amplifier are formed in epitaxial silicon simultaneously acquiring the same temperature coefficient. This substantially reduces the variations in the overall sensitivity with temperature changes. However, for certain applications it is desired to have greater stability with temperature as well as with changes in supply voltage. The later problem is partly accounted for by the on-board-voltage regulator in the Anselmo et al patent.
It is an object of the present invention to provide a ferromagnetic material detector having a high degree of stability in output voltage as a function of temperature and supply voltage.
It is a further object of this invention to provide such a detector having an output voltage with a low value DC component.